Preheat the oven to 180°C, fan 160°C, gas mark 4. Coat and cook the chicken breasts with the herby mixture in the bag, according to pack instructions.
Meanwhile, place the Maris Piper potato pieces in a saucepan and bring to the boil. Cook for 5 minutes then add the sweet potato and cook for a further 5 minutes or until tender.
Remove from the heat, then drain and roughly mash the potatoes together. Add the pepper and the buttery spread and continue to mash until smooth. Set aside and keep warm.
Meanwhile, heat the oil in a frying pan and quickly sauté the pak choi until wilted.
Remove the chicken from the oven, leave to cool for a minute then cut the bag open and gently tip the contents into a dish. Slice the chicken breasts on the diagonal and serve on top of the mash with the pak choi on the side.